For time-of-flight (TOF) positron emission tomography (PET) imaging detectors with good timing resolution are required for improving image quality. One of the effects degrading the timing resolution is the statistical fluctuation of the number of scintillation photons and their transportation inside the crystal.
A depth-of-interaction (DOI) correction is important to achieve sub-250 ps timing resolution in TOF-PET detectors utilizing currently available fast scintillators such as LSO (Lutetium Oxy-orthosilicate) or LYSO (Lutetium Yttrium Oxy-orthosilicate) and fast photosensors. Due to optical photon dispersion (different path lengths) in the scintillator crystal, timing resolution deteriorates with increasing crystal length; at the same time, a long (about 20-30 mm) crystal is needed for good detection efficiency (high stopping power).
Traditionally the DOI correction could be implemented by using a dual-ended readout, and/or using a multiple-layer scintillator. However, the timing resolution is significantly worse for both of these approaches due to lower light collection efficiency. Additionally, the cost associated with a complex electronic logic and detector package (additional scintillator and/or photodetector) has mostly precluded their commercialization for next-generation (<250 ps) clinical PET scanners.